1. Field
One embodiment of the present invention relates to a printed wiring board structure in which a semiconductor package having a semiconductor chip loaded on a substrate is mounted on both surfaces of a printed wiring board.
2. Description of the Related Art
A circuit board mounted with a large-sized semiconductor package forming a CPU and its peripheral circuits is provided as a main component in a housing of an electronic apparatus such as a personal computer. The large-sized semiconductor package has some tens of millimeters square.
This kind of circuit board used for the electronic apparatus such as the personal computer requires means for protecting a mounting surface of the semiconductor package. Specifically, there is a need of protecting the mounting surface from warp and deformation of a substrate and from stress applied by impact and vibration given externally.
For example, the following method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-271014 has been known as means for protecting a solder joint part of a semiconductor component mounted on a substrate. According to the method of mounting electronic components, a semiconductor device is fixed to the substrate using an under-fill resin as a reinforcement means.
The reinforcement means using the foregoing under-fill resin is applied to the circuit board mounted with a large-sized semiconductor package having some tens of millimeters square. In this case, the following problem arises. Specifically, self-heating caused by a circuit operation of the semiconductor package occurs. For this reason, thermal expansion of the reinforcement material filled as the under-fill between the semiconductor package and the substrate is repeated every time the semiconductor package is operated. The foregoing thermal expansion applies excessive stress to the solder joint portion of the semiconductor package. In particular, a circuit board mounted with a large-sized semiconductor package such as BGA and LGA having the solder joint portion array on the package backside has the following problem. Namely, stress is concentrated on the corner portion of a rectangular package, and thereby, the circuit of the solder joint portion is broken down. This problem further remarkably appears when coefficient of thermal expansion of the reinforcement material filled as under-fill differs from those of the semiconductor package and the substrate. In addition, there is a problem that rework is difficult because the entire mounting surface of the large-sized semiconductor package is fixedly bonded to the substrate.
On the other hand, with multi-function and high function of electronic apparatus, a circuit board applied to the electronic apparatus requires higher mounting density of wirings and mounting components. In order to meet the foregoing requirements, the following various printed wiring board structures have been proposed. According to one conventional printed wiring board structure, semiconductor packages such as BGA and LGA, each loading a semiconductor chip on a substrate, are mounted in an overlap manner on both surfaces of a printed wiring board. This kind of printed wiring board structure has the following problem. An overlapping ratio of semiconductor packages mounted on both surfaces of the printed wiring board gives a negative influence to the connection reliability in the printed wiring board. If the overlapping ratio increases, large stress-strain is applied in accordance with the foregoing increase; as a result, connection reliability is reduced. In other words, when the foregoing overlapping ratio is reduced, improvement of the connection reliability is performed. However, reducing the overlapping ratio is a factor of reducing the mounting density of wirings and components. As a result, there is a problem that a circuit board is made into a large size.
According to one aspect of the present invention, there is provided a printed wiring board structure comprising:
a printed wiring board having first and second component mounting surfaces at front and back sides thereof, respectively, each for mounting a semiconductor package loading a semiconductor chip loaded on a substrate as a mounting component;
a first semiconductor package mounted on the first component mounting surface; and
a second semiconductor package mounted on the second component mounting surface,
wherein the first and second semiconductor packages have a positional relationship such that the substrates are partially overlapped via the printed wiring board, and the semiconductor chips are not overlapped.